Nuuvem is a Brazilian digital games store and they are offering some free Steam keys, an account with them is required for this giveaway.
1. Go here
2. Login / Register
3. Click on 'Add'
4. Click on 'Checkout' at the cart
5. Confirm the order
6. Your Steam key should be sent to your e-mail
There are other freebies you can get from Nuuvem here (Relic Hunters Zero gives you Steam card drops)